The goal is to explore the connectional organization of the mammalian superior colliculus at both the light microscopic and ultrastructural levels. The stratum griseum intermediale (SGI) will be examined in depth. Cells within this layer fire prior to saccadic eye movements and project upon brainstem cell groups involved in oculomotor control. SGI neurons also innervate brainstem and spinal cord regions involved in head and body movements. The SGI consists of three sublayers (dorsal, middle and ventral) which are targeted by distinct patterns of afferents, the great majority of which distribute in patches. In order to shed some light upon both the functions of the individual SGI sublayers, and to better understand the significance of their patchy inputs, several lines of investigation will be pursued. A major portion of proposed studies will focus upon the organization of the middle sublayer of the SGI. This sublayer has been shown to consist of two connectionally distinct domains, one targeted by overlapping patchy inputs from the substantia nigra pars reticulata (SNpr) and the pedunculopontine tegmental nucleus (PTN), and the other innervated by overlapping projections from three somatosensory related regions (spinal trigeminal, fourth; SIV, and fifth; SV, cortical areas). Double label anterograde transport methods will be used to define the specific domain targeted by seven additional patchy afferents to the middle sublayer. Double label anterograde transport methods will also be combined with electron microscopic approaches in order to examine overlapping inputs within the "somatosensory domain" of the middle sublayer of the SGI. In particular, the morphology and synaptic relationships of two somatosensory inputs within a single patch will be examined. The prominent cholinergic input to the SGI will also be examined. Each of the three sublayers of the SGI receives a patchy cholinergic projection which arises almost exclusively from the PTN and the laterodorsal tegmental nucleus (LDTN). Anterograde transport data collected during the last grant period suggest that different regions of the PTN project to different sublayers of the SGI. Thus the organization of the PTN and LDTN projections to the SGI will be studied in order to determine the specific region(s) of the caudal cholinergic cell group which projects upon each of the three sublayers of the SGI. The final goal of the present proposal centers upon the global nigrotectal projection in the cat. This pathway has been found to innervate each of the three sublayers of the SGI, as well as the deep gray layer (stratum griseum profundum; SGP), the superficial gray layer (stratum griseum superficiale; SGS) and the contralateral colliculus. The present experiments will use the electron microscope to examine the morphology and synaptic relationships of nigral terminals comprising each of the three nigrotectal "channels". Similar studies will also examine nigral terminals within the SGP, the SGS and the contralateral colliculus.